Transcript Balancing Chemical Reactions - Xavier High School

Balancing Chemical
Reactions
Objectives
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Write equations describing chemical
reactions using appropriate symbols
Write balanced chemical equations when
given the names or formulas of the
reactants and products in a chemical
reaction
Define chemical equation, catalyst,
aqueous solution, skeleton equation,
coefficients, and balanced equation
Chemical Equations
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Chemical equations – using chemical formulas to
write equations
Reactants (left side of arrow)
Products (right side of arrow)
Arrow means yields, gives, or reacts to produce
Reactants  Products
Catalyst (a substance that speeds up the rate of
the reaction but that is not used up in the
reaction) should be written above the arrow
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Can indicate the physical state of a
substance in the equation by putting a
symbol after each formula
Solid – (s)
Liquid – (l)
Gas – (g)
Aqueous solution: a substance dissolved
in water – (aq)
Refer to Table 8.1 on page 206 for
explanations of other symbols used in
chemical equations
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Iron reacts with oxygen to produce rust.
Iron + oxygen  iron(III) oxide
Hydrogen peroxide reacts to form water and
oxygen. (Bubbles=oxygen gas)
Hydrogen peroxide  water + oxygen
Residential heating
Methane + oxygen  carbon dioxide +
water
Green color on the Statue of Liberty (copper
exposed to moist air)
Copper + carbon dioxide + water  basic
copper(II) carbonate
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Glucose is fermented by yeast to form ethanol
and carbon dioxide
Glucose  ethanol + carbon dioxide
Yeast should be written above the arrow
because it is neither a reactant nor a product.
Plants carry out photosynthesis - the creation of
glucose and oxygen from carbon dioxide, water,
and sunlight
Carbon dioxide + water  glucose + oxygen
Sunlight should be written above the arrow
because it is neither a reactant nor a product.
Skeleton Equation
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A chemical equation that does not indicate the
relative amounts of the reactants and products
involved in the reaction
Examples:
a. Fe(s) + O2(g)  Fe2O3(s)
b. H2O2(aq)  H2O(l) + O2(g)
Manganese(IV) oxide is a catalyst, so MnO2
should be written above the arrow.
See Figure 8.3 on page 205 in text.
Link to Human Physiology
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Hydrogen peroxide decomposes to oxygen
and water when it comes into contact with
blood. The enzyme catalase is a catalyst
that contains an iron(II) ion. When
hydrogen peroxide is poured on a cut, it
reacts with the iron(II) ions of catalase,
thus releasing energetic oxygen atoms
that produce the antiseptic effect of
hydrogen peroxide.
Write the skeleton equation for
hydrogen peroxide linked to human
physiology
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Answer: H2O2(aq)  H2O(l) + O2(g)
Iron(II) ion is a catalyst, so Fe2+ should
be written above the arrow.
This is an example of a decomposition
reaction : a single compound is broken
down into two or more products
Link to Art
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Hydrogen peroxide can also be used to
restore the clarity of old paintings.
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Lead-based paints darken with time (PbS).
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Hydrogen peroxide converts PbS to PbSO4.
Write a Skeleton Equation
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Solid sodium hydrogen carbonate reacts
with hydrochloric acid to produce aqueous
sodium chloride, water, and carbon dioxide
gas. Include appropriate symbols.
1. Write the correct formula for each
substance in the reaction.
2. Separate the reactants from the
products.
3. Indicate the physical state of each
substance.
Answer
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NaHCO3(s) + HCl(aq)  NaCl(aq) +
H2O(l) + CO2(g)
A Balanced Equation
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An equation that gives the correct quantity
of each reactant and product
Coefficients (numbers placed in front of
the symbols) are used
Must obey law of conservation of mass:
Each side of the equation has the same
number of atoms of each element
Example: A standard bicycle is composed
of one frame, two wheels, one handlebar,
and two pedals
F + 2W + H + 2P  FW2HP2
Rules for Balancing Equations
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1. Determine the correct formulas for all of the
reactants and products. In some cases, also list
in parenthesis the physical state of matter.
2. List reactants on the left side of the arrow
(Use plus sign (+) when there is more than one
reactant)
3. List the products on the right side of the
arrow (Use plus sign (+) when there is more
than one product)
4. Steps 1-3 provide a skeleton equation.
(Note: Sometimes this is also the balanced
equation. For example: C + O2  CO2)
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5. Count the number of atoms of each element
in the reactants and products. For simplicity, a
polyatomic ion appearing unchanged on both
sides of the arrow is counted as a single unit.
6. Balance the elements one at a time by using
coefficients. DO NOT CHANGE THE
SUBSCRIPTS.
7. Check each atom or polyatomic ion to be
sure that the equation is balanced.
8. Make sure that all the coefficients are in the
lowest possible ratio that balances.
Problem: Hydrogen and oxygen react to
form water. Write a balanced equation.
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Reactants: H2(g) + O2(g)
Products: H2O(l)
H2(g) + O2(g)  H2O(l)
Count the atoms
Left side
Right side
H–2
H–2
O–2
O–1
Use coefficient to get 2 oxygen on the right side:
H2(g) + O2(g)  2 H2O(l)
Left side
Right side
H–2
H–4
O–2
O–2
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Need 4 hydrogen atoms, so place a
coefficient of 2 in front of H2
2H2(g) + O2(g)  2 H2O(l)
Left side
Right side
H–4
H–4
O–2
O–2
Check number of atoms
Check that the coefficients are in the
lowest possible ratio
The equation is balanced
Problems
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1. Balance the following equations:
a. SO2 + O2  SO3
b. Al + O2  Al2O3
2. Rewrite the word equation as a
balanced chemical equation:
Aluminum sulfate and calcium
hydroxide react to form aluminum
hydroxide and calcium sulfate.
Answers
1a) 2SO2 + O2  2SO3
1b) 4Al + 3O2  2Al2O3
2)
Word equation to balanced chemical
equation:
Al2(SO4)3 + 3Ca(OH)2  2Al(OH)3 + 3CaSO4
Practice Problems
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1. __NaCl + __BeF2
__NaF + __BeCl2
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2. __FeCl3 + __Be3(PO4)
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3. __AgNO3 + __LiOH
__AgOH + __LiNO3
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4. __CH4 + __O2
__CO2 + __H2O
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5. __Mg + __Mn2O3
__MgO + __Mn
2
__BeCl2 + __FePO4
Types of Chemical Reactions
Objectives:
 1. Identify a reaction as combination,
decomposition, single-replacement,
double-replacement, or combustion
 2. Predict the products of combination,
decomposition, single-replacement,
double-replacement, and combustion
reactions
Classifying Reactions
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For combination (synthesis: combination of parts
into a whole) and decomposition, compare the
number of reactants and products
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For combustion, check for oxygen
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For single- and double-replacement, look for a
cation swap or the formation of a precipitate
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Not all chemical reactions fit uniquely into only
one of these classes
Combination Reactions
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Two or more substances combine to form
a single substance
Reactants are usually either two elements
or two compounds
The product is always a compound (Can
be an ionic compound or a molecular
compound)
Some nonmetal oxides react with water to
produce an acid (hydrogen ions in
aqueous solution)
Some metal oxides react with water to
form a base (hydroxide ions)
Worksheet Handout
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Refer to the worksheet handout.
Identify the combination reactions.
Decomposition Reactions
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A single compound is broken down into
two or more products
The products can be any combination of
elements and compounds
Most decomposition reactions require
energy in the form of heat, light, or
electricity
Extremely rapid decomposition reactions
that produce gaseous products and heat
are often the cause of explosions
Worksheet Handout
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Refer to the worksheet handout.
Identify the decomposition reactions.
Single-replacement Reactions
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Also called single-displacement reactions
One element replaces a second element in
a compound
Can be compared to partners cutting in on
each other at a dance: A person who is
alone approaches a dancing couple and
cuts in…that person replaces one member
of the couple, who is now left alone
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Whether one metal will displace another
metal from a compound can be
determined by the relative reactivities of
the two metals. (Memorize the symbols
and activity series of metals on page 217.)
A reactive metal will replace any metal
listed below it in the activity series
Examples:
Iron will displace copper from a copper
compound in solution.
Magnesium does not replace lithium
from aqueous solutions of their
compounds.
Refer to Table 8.2 on page 217
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Will magnesium displace zinc from a zinc
compound in solution?
Will magnesium displace silver from a silver
compound in solution?
Important Note:
1. Metals from lithium to lead will replace
hydrogen from acids.
2. Metals from lithium to sodium will also
replace hydrogen from water.
Single-Replacement (cont’d)
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A nonmetal can also replace another nonmetal
from a compound
This replacement is usually limited to the
halogens (Group 7A):
F2 (most activity)
Cl2 .
Br2 .
I2 (least activity)
The activity of the halogens decreases as you go
down group 7A on the periodic table
Worksheet Handout
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Refer to the worksheet handout.
Identify the single-replacement reactions.
Double-replacement Reactions
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Involves an exchange of positive ions
between two reacting compounds
Often characterized by the production of a
precipitate (ppt.-insoluble substance that
“falls out” of a solution)
Product may be a gas that “bubbles” out
of the mixture
Product may be a molecular compound,
such as water
Worksheet Handout
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Refer to the worksheet handout.
Identify the double-replacement reactions.
Combustion Reactions
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An element or a compound reacts with
oxygen, often producing energy as heat
and light
Commonly involve hydrocarbons
(compounds of hydrogen and carbon)
The complete combustion of a
hydrocarbon produces carbon dioxide and
water
If the supply of oxygen during a reaction
is insufficient, combustion will be
incomplete
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During incomplete combustion, elemental
carbon and toxic carbon monoxide may be
additional products
Reaction between some elements and
oxygen
Example: Both magnesium and sulfur will
burn by reaction with oxygen
Refer to worksheet handout.
Identify the combustion reactions.
Make a Chemistry Foldable
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1. Fold a sheet of notebook paper to the red
margin line.
2. Using scissors, cut the folded section into five
equal parts.
3. Label each section with the name of one of
the five types of reactions.
4. Open each flap and put in three
characteristics and one example (include
balanced equation).
5. Write the title : Types of Chemical Reactions
on the top of the sheet. Add your name and
class.
6. Use the chemistry foldable as a study guide.
Predicting Products of a Chemical
Reaction
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Recognize the possible type of reaction that
the reactants can undergo
Some reactions do not fit any one of the five
general types (Example: redox reactions)
Oxidation-reduction (redox) reactions will be
discussed during the second semester
OIL RIG – oxidation is the loss of electrons
and reduction is the gain of electrons
LEO the lion says GER – loss of electrons
is oxidation and gain of electrons is reduction
Reactions in Aqueous Solution
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Objectives:
1. Write and balance net ionic equations
2. Use solubility rules to predict the
precipitate formed in double replacement
reactions
Net Ionic Equations
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Most ionic compounds dissociate, or separate,
into cations and anions when they dissolve in
water.
Refer to question #21 on the worksheet
handout. Use this equation to answer #22 on
the worksheet handout.
Write a complete ionic equation that shows
dissolved ionic compounds as their free ions.
Eliminate ions that do not participate in the
reaction by canceling ions that appear on both
sides of the equation. These are called spectator
ions.
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Ions that are not directly involved in a reaction
are called spectator ions.
Rewrite the equation, leaving out the canceled
spectator ions.
Balance the atoms and the charges of the ions.
(In this case, the number of atoms and the net
ionic charge on each side of the equation is zero
and it is therefore balanced.)
A net ionic equation indicates only those
particles that actually take part in the reaction.
Record your answer to #23 on the worksheet
handout.
Practice Problem
Write a balanced net ionic equation for the following
reaction:
Pb(s) + AgNO3 (aq)  Ag (s) + Pb(NO3)2 (aq)
Answer:
1. The nitrate ion is the spectator ion.
2. The number of atoms balance, but the charges on
the ions do not balance.
3. Place a coefficient 2 in front of Ag+ (aq) to balance
the charges.
4. A coefficient of 2 in front of Ag (s) rebalances the
atoms.
5. Pb(s) + 2Ag+ (aq)  2Ag (s) + Pb2+ (aq) is the
balanced net ionic equation
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Predicting the Formation of a
Precipitate
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Use the general rules for solubility of ionic
compounds (Table 8.3 on page 227)
Examples:
1. Sodium nitrite will not form a
precipitate because alkali metal salts and
nitrate salts are soluble (Rules 1 and 2)
2. Rule 3 (Exceptions) indicates that
barium sulfate is insoluble and therefore
will precipitate.
Solubility Rules for Ionic Compounds
Compounds
Solubility
Exceptions
1. Salts of alkali metals
and ammonia
Soluble
Some lithium
compounds
2. Nitrate salts and
chlorate salts
Soluble
Few Exceptions
3. Sulfate salts
Soluble
Compounds of Pb, Ag,
Hg, Ba, Sr, and Ca
4. Chloride salts
Soluble
Compounds of Ag and
some compounds of Hg
and Pb
5. Carbonates,
Most are insoluble
phosphates, chromates,
sulfides, and
hydroxides
Compounds of the
alkali metals and of
ammonia
Practice Problem
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Identify the precipitate formed and write
the net ionic equation for the reaction of
aqueous potassium carbonate with
aqueous strontium chloride.
1. Write the reactants showing each as
dissociated free ions. Balance the charges.
2. Using solubility rules, look at possible
new pairings of cation and anion that give
an insoluble substance.
3. Eliminate the spectator ions and write
the net ionic equation.
Answer
1. Reactants as dissociated free ions
2K+ (aq) + CO32- (aq) + Sr2+ (aq) + 2Cl- (aq)
Charges must be balanced to equal 0.
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2. Of the two possible combinations, KCl is soluble
(Rules 1 and 4) and SrCO3 is insoluble (Rule 5)
3. The net ionic equation must be balanced for the
number of atoms of each element and the charges
on the ions.
Sr2+ (aq) + CO32- (aq)  SrCO3 (s)
Note: Ignore Sample Problem 8-11 on page 228.
There is a textbook error.